Element Selenium, Se, Non Metal
History| Selenium was first identified in 1817 by the Swedish chemist Jakob Berzelius (1779-1848) after analyzing an impurity that was contaminating the sulfuric acid (H2SO4). Berzelius and his colleague Johann Gottlieb Gahn (1745-1818) were studying a method of producing sulphuric acid in lead cameras "We observed residues of a substance, partially red and partially light-brownish, with a very intense scent in the bottom of the camera. Such scent, according to Klaproth, is the sign of presence of tellurium, so at first we thought it was tellurium. Gahn noticed that in mines of Falun, were sulphur is collected, similar scent of tellurium is also often felt. Interested and hoping to discover a new metal, I commenced to investigate substance of the residues. However, more careful attempts to separate tellurium, comparing with the analysis of burnt ores from Falun, revealed that there were no residues of this element, in spite of its identical properties. It was a new substance and, following the identity of properties, was given the name Selenium, term that derives from the Greek ∑εληνη (Moon), since Klaproth had named tellurium for the Earth." |
OccurrenceAverage crustal abundance is 1.4x10-5% by mass, in sea waters 4x10-3mg/l. Over 50 selenium minerals are known such as native selenium, berzelianite (copper selenide), naumannite Ag2Se, chalcomenite CuSeO3 x 2 H2O. Selenium concentration in pyrite (iron sulphide), galena (lead sulphide), bismuthinite (bismuth sulphide) and other sulphides, as well as in volcanic sulphur, reaches several per cents.
Selenium is most commonly produced from selenide in many sulphide ores, such as those of copper, Silver, or lead.
Selenium is a very rare trace element. It history in the Earth's crust is closely related to sulphur history. Selenium has concentration ability and, despite the low abundance, creates 38 minerals, including selenide, selenate and selenite. Isomorphic impurities of selenium in sulphides and native sulphur are typical.
Selenium energetically migrates in biosphere. Rocks, volcanic fumes and thermal waters are the main source of selenium concentration in living organisms. For this reason soils of contemporary and ancient volcanic areas are enriched by selenium, reaching in clays and slates up to 6 x 10-5%. In 1952 Abdullaev, the young Soviet scientists, who later became the President of Azerbaijan Academy of Science, noticed, that the spectral sensibility of human eye matched with spectra of selenium used in photoelectric cells. Thus hypothesis arose that selenium has its role in light energy transformation into the electrical signals energy, which is responsible for vision.
Long time later this hypothesis had been confirmed by discovering selenium in retina. The concentration in human eye is 7 µg, 100 times less than in eyes of sharp-sighted eagle, 780 µg. This discovery was proved in experiments with rabbit's vision.
Selenium has a biological role also in enzymatic reaction; its compounds are responsible for protection from irradiation injury. Selenium needs level is, however, not so high, approximately 50...100 µg per kg of food. |
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